1. Field of the Invention
The present invention relates to a solid state imaging device and a method for producing the same, and more specifically to a solid state imaging device including a smoothing layer on a light shielding film and a method for producing the same.
2. Description of the Background Art
In general, a solid state imaging device includes a pixel section having a plurality of pixels arranged in a matrix, and each pixel includes a light receiving section in a main surface portion of a semiconductor substrate. The light receiving section is constructed to output an electric signal in accordance with an amount of light incident thereon. A light shielding film is provided on the light receiving section for preventing light from being received by elements other than the light receiving section. The light shielding film provided on each light receiving section has an opening for guiding the light to the light receiving section. For such a light shielding film, a metal layer having a high light shielding property such as, for example, a tungsten film is used. However, a metal layer is likely to scatter the incident light at the surface thereof, and the light scattering ratio is different among the plurality of pixels due to the difference in the surface roughness. This causes a variance in the light collecting state among the light receiving sections, which tends to cause sensitivity non-uniformity in the solid state imaging device.
Under such circumstances, techniques for subjecting the surface of the light shielding film to various types of processing so as to alleviate the sensitivity non-uniformity of the solid state imaging device have been proposed. For example, Japanese Laid-Open Patent Publications Nos. 9-232552 and 2004-95895 disclose a method of oxidizing a surface of the light shielding film simultaneously with forming a smoothing layer which covers the light shielding film.
Hereinafter, a structure of a CCD (Charge Coupled Device), as an example of a solid state imaging device 400 including a light shielding film having an oxidized surface, will be described with reference to FIG. 4 and FIGS. 5A through 5E. FIG. 4 is a cross-sectional view of a pixel of the solid state imaging device 400. The solid state imaging device 400 is produced by the steps shown in FIGS. 5A through 5E.
FIGS. 5A through 5E are cross-sectional views showing the steps of production of the solid state imaging device 400 shown in FIG. 4. FIG. 5A is a cross-sectional view showing a state where a tungsten film 7a, which is to be a light shielding film 7 are formed on other elements which are formed on a semiconductor substrate 1. Such a structure is obtained as follows. First, a photodiode 2 and a charge transfer section 3 are formed in a main surface portion of the semiconductor substrate 1 by, for example, ion implantation. Next, a gate insulating film 4 is deposited on a main surface of the semiconductor substrate 1 by thermal oxidation or CVD (Chemical Vapor Deposition). When the deposition of the gate insulating film 4 is completed, a polysilicon layer is deposited by CVD and patterned into a required shape (not shown) by, for example, photolithography and dry etching, thereby forming a transfer electrode 5. Next, an inter-layer insulating film 6 formed by silicon oxide is formed by oxidation and CVD so as to cover the transfer electrode 5 and the gate insulating film 4. Then, the tungsten film 7a to be the light shielding film 7 is formed on the resultant structure by sputtering or CVD.
FIG. 5B is a cross-sectional view showing a state where a resist pattern 11 is formed in order to obtain a desired shape of the light shielding film 7. The resist pattern 11 is formed as follows. First, a surfactant is applied on the tungsten film 7a. Next, a resist is applied thereon to form a resist layer, and the resist layer is patterned by exposure and development, thereby forming an opening in the resist layer above the photodiode 2. Thus, the resist pattern 11 is formed.
FIG. 5C is a cross-sectional view showing a state where the tungsten film 7a is patterned. The tungsten film 7a is patterned by dry-etching using the resist pattern 11 as a mask. Thus, the light shielding film 7 patterned to have a desired shape is obtained. Then, the resist pattern 11 is removed.
Next, a smoothing layer 21 is formed on the light shielding film 7 by CVD. Before forming the smoothing layer 21, however, a surface of the light shielding film 7 is oxidized. FIG. 5D is a cross-sectional view showing a state where the surface of the light shielding film 7 is oxidized. The light shielding film 7 is oxidized in a chamber which is also used for CVD performed forming the smoothing layer 21. Ozone (O3) gas or oxygen (O2) gas is introduced into the chamber, thereby exposing the wafer to the gas atmosphere. Thus, the light shielding film 7 is gradually oxidized from the surface thereof, resulting in the formation of a tungsten oxide film 20. Since the tungsten oxide film 20 provides a low light reflectance, the scattering ratio of the incident light at the surface of the light shielding film 7 can be reduced.
FIG. 5E is a cross-sectional view showing a state where the smoothing layer 21 is formed. After the O3 gas or the O2 gas is expelled from the chamber, the smoothing layer 21 is formed by introducing tetraethoxysilane (TEOS) gas to the chamber. The obtained smoothing layer 21 is subjected to a smoothing treatment, so that lenses (not shown) for collecting the incident light to the light receiving section can be formed on the surface. In this manner, the solid state imaging device 400 shown in FIG. 4 is obtained.
The solid state imaging device 400 having the above-described structure has the following problems. First, the tungsten oxide film 20 easily transmits light. Due to such a property, when the tungsten oxide film 20 is formed by oxidizing the light shielding film 7 from the surface thereof, the light shielding film 7 formed of tungsten becomes thinner, which changes the effective light shielding ratio of the light shielding film 7. In order to maintain the original effective light shielding ratio of the light shielding film 7, it is necessary to form the tungsten film 7a to be thicker than actually necessary in consideration of the reduction in the thickness by the formation of the tungsten oxide film 20. However, it is actually difficult to expect how much the thickness of the light shielding film 7 will be reduced, and in addition, an increase in the thickness of the light shielding film 7 prevents size reduction of pixels.
Second, when the surface of the light shielding film 7 was oxidized at a temperature of about 500° C. as described in Japanese Laid-Open Patent Publication No. 2004-95895 in a CVD chamber as described above, it was found to be difficult to form the tungsten oxide film 20 with a uniform thickness. This is considered to be due to the oxidation temperature of the light shielding film 7 formed of tungsten. In general, a tungsten film is easy to be oxidized, and so is oxidized to a very small degree even at room temperature. It is considered that a tungsten film starts to be oxidized at about 400° C. and the oxidation rapidly proceeds at about 700° C. The reason why the thickness of the tungsten oxide film 20 is non-uniform is considered to be that an oxide film is not sufficiently formed at a temperature of about 500° C.
In the case where the tungsten oxide film 20 having a non-uniform thickness is formed on the surface of the light shielding film 7, the light reflectance is lower in an area where the tungsten oxide film 20 is thicker and is higher in an area where the tungsten oxide film 20 is thinner. In the case where the light shielding film 7 formed of tungsten has areas having tungsten oxide formed thereon and areas having no tungsten oxide formed thereon as a result of non-uniform oxidization, the levels of these two different areas are different; i.e., the surface of the light shielding film 7 has convexed areas and concaved areas. As a result, the light scattering ratio is different between these two different areas (i.e., between the surface of the light shielding film 7 and the surface of the tungsten oxide).
Therefore, the solid state imaging device 400 has a variance in the light collecting state, i.e., sensitivity non-uniformity due to the non-uniform reflectance in the vicinity of the opening 8, in each light receiving section. Even if each light receiving section has a variance in the light collecting state, as long as the variance is uniform among the plurality of pixels, the sensitivity non-uniformity is small in the entire pixel section and causes little problem. However, in the solid state imaging device 400 having the above-described structure, the scattering ratio at the surface of the light shielding film 7 is different among the plurality of pixels, and therefore the sensitivity non-uniformity is large in the entire pixel section. It is conceivable to oxidize the light shielding film 7 at a temperature as high as 700° C. in order to obtain the tungsten oxide film 20 having a uniform thickness. However, such a treatment causes deterioration in the light receiving section or the like and is not preferable.